A. Technical Field
The present invention relates generally to x-ray tube technology. More specifically, the present invention relates to geometrical and electrical biasing improvements for reducing the size of the focal spot of electrons striking the tube anode.
B. Background Art
Conventional diagnostic use of x-radiation includes the form of radiography, in which a still shadow image of the patient is produced on x-ray film, and fluoroscopy, in which a visible real time shadow light image is produced by low intensity x-rays impinging on a fluorescent screen after passing through the patient.
In a typical x-ray tube, electrons are generated of a filament coil heated to thermionic emission. The electrons are accelerated as a beam from a tube cathode through an evacuated chamber defined by an envelope, toward a tube anode. When the electrons strike the anode with large kinetic energies, and experience a sudden deceleration, and x-radiation is produced. The x-ray tube assembly is contained in a housing which includes a window transmissive to x-rays, such that radiation from the anode passes through the window toward a subject undergoing examination or treatment.
Most x-ray tube designs employ filaments as a source of electrons with which to bombard the tube anode. A filament is a coil of wire which is electrically energized so that electrons are thermionically emitted from the filament and accelerated toward the anode due to a very large DC electrical potential difference between the cathode and the anode. Often this electrical potential difference is of the order of 150,000 volts, (.+-.75,000 volts to ground) necessitating elaborate means for isolating the electrical elements and connectors which are used to apply such an amount of voltage.
The cathode filament is thermionically energized with a relatively low voltage (on the order of 10 volts) and high current AC signal. Although the peak-to-peak magnitude of the energization signal for the filament is low, the reference, or average, potential of the filament is, in the instance described here, about -75,000 volts DC. Stated another way, the voltage on the filament with respect to ground is up to -75,000 volts, plus or minus a low level alternating current signal needed to boil off electrons from the filament. At these high voltages, the filament input must be heavily insulated to prevent arcing. The insulation typically is in the form of high voltage cabling and connectors, which are expensive and complex in design. In fact, any electrically conductive element or member electrically contacting or coupled to the cathode must be provided with this type of expensive isolative means.
It is desirable to focus the electrons so that the focal spot at which they strike the anode is as small as possible. This causes the resulting x-rays to emanate from as small a source as possible, to minimize diffusion in the x-ray generated image.
A trend toward shorter x-ray exposure times in radiography has dictated a need for greater intensity of radiation and hence higher electron currents. Attempts to increase the intensity, while, at the same time, decreasing the focal spot size, can cause overheating of the x-ray tube anode.
One way to control the size of the focal spot of the electrons on the anode is to mount the cathode filament on a cathode focusing or support cup member. It has been proposed to apply an electrical bias voltage between the cathode cup member and the filament in order to control, to some extent, the size of the focal spot.
Cathode cup and filament arrangements for controlling the size and shape of the electron focal spot on the tube anode are discussed in U.S. Pat. Nos. 4,685,118, issued on Aug. 4, 1987, and 4,799,248, issued Jan. 17, 1989, both to Furbee, et al., and assigned to the assignee of the present invention. These two U.S. Patents. are hereby expressly incorporated by reference in their entirety.
It is known in the art to apply a DC bias between the filament and the cup of an x-ray tube in order to reduce unwanted "wings", or diffused areas, appearing as part of the x-ray focal spot. The focused electron beam, at the focal spot, has a generally rectangular shaped distribution, the length of which corresponds to the filament length and the width of which comprises a central portion produced from electron emission from the front of the filament, and peripheral portions or "wings" produced from electron emission from the back or side portion of the filaments. One proposal for eliminating wings is discussed in U.S. Pat. 4,764,947, issued on Aug. 16, 1988, to Lesensky and entitled "Cathode Focusing Arrangement".
In order to electrically bias the filament/cup arrangement to sufficiently eliminate the "wings" as proposed by Lesensky, however, the DC voltage has had to be on the order of a hundred volts or more. Voltages of this magnitude require the provision of rather complex DC power sources, either independent sources, or means for deriving the DC voltage from the x-ray filament AC power supply. A complicating factor is that these power supplies must be isolated to the kV level at which the cathode and filament respectively reside during x-ray tube operation.
Proposals have also been made for governing the mechanical geometrical relationships of the various parts of an x-ray tube, particularly the filament and focusing cup, to eliminate the "wings" on the x-ray focal spot, without the need for any DC filament/cup biasing at all. See the above identified '947 patent, column 8, line 34 et. seq. When one so modifies an x-ray tube's geometry to eliminate wings, however, emission levels from the filament have been seriously reduced. This means that, for a given desired radiation emission level, the filament must be operated at a higher temperature than would otherwise be the case. The higher temperatures necessitated by the same mechanical design which helps eliminate the wings also, unfortunately, shortens filament life, and therefore x-ray tube life, often to an unacceptable degree.
It is a general object of this invention to increase the emission of the filament, at a given filament operating temperature, without increasing the width of the focal spot, specifically by eliminating the undesirable dispersion of the x-ray tube electron beam, which causes the "wings", by the use of simple and inexpensive means.